Unit 17: Selection & Evolution (JW)

Question 1

Explain what phenotypic variation is caused by

Answer 1

Phenotypic variation = genetic / environmental factors OR combo of genetic + envi factors

Question 2

Explain, using examples, how the environment may affect the phenotype of individual organisms.

Answer 2

Phenotype results from interaction of genotype & environment

1) Environment may modify gene expression = results in continuous variation e.g. size, mass, height
due to food, nutrients.. etc.

2) Environment may switch on gene
e.g. UV light and melanin production

3) Environment may induce mutation e.g. UV light

environment effect usually greater on polygenes

Question 3

Define variation

Answer 3

differences that exist between individuals of a species

Question 4

explain what is meant by discontinuous variation

Answer 4

e.g. blood groups
qualitative differences in phenotypes
discrete & categoric
Feature either present / absent
no range / intermediates
does not show normal distrib curve

Question 5

Explain what is meant by continuous variation

Answer 5

e.g. mass, height
quantitative differences in phenotypes
not discrete
range of phenotypes
Polygenic
normal distribution

Question 6

explain the genetic basis of discontinuous variation

Answer 6

solely due to genetic factors
genotype = phenotype
Different alleles at a single gene locus have a large effect on the phenotype
e.g. different alleles at the F8 gene locus dictate whether or not normal Factor VIII is produced and whether the individual has the condition haemophilia

Question 7

explain the genetic basis of continuous variation

Answer 7

different alleles at single gene locus have small effects on phenotype
different genes may have additive effect on phenotype
environment has effect on phenotype (interaction btwn genes + environment)
phenotype = genotype + environment

Question 8

Why t-test suitable?

Answer 8

Compare 2 means of 2 sets of data (paired set of data)
Continuous data
Normal distribution

SD are approx the same
2 samples have < 30 values each

Question 9

Describe and explain why natural selection occurs

Answer 9

Natural selection states organisms change overtime.

Organisms produce many offspring, more than required to maintain population. These offspring, within species, will have genetic variation due to random mutation.

When they compete for resources such as limited food, this counts as selection pressure. Some individuals may possess advantageous alleles that randomly mutated.

These fitter individuals are likely to survive, reproduce, pass on the advantageous alleles to offspring, change in allele frequency within population.

Question 10

Explain how environmental factors can act as stabilising forces of natural selection

Answer 10

2 extremes selected against
median selected for
no change in environment

Question 11

Explain how environmental factors can act as disruptive forces of natural selection

Answer 11

2 extremes are selected for
median is selected against
bimodal distribution
diversifying selection
maintains genetic diversity

Question 12

Explain how environmental factors can act as directional forces of natural selection

Answer 12

1 of the extremes selected for

Question 13

4 processes that affect allele frequencies

Answer 13

Natural selection
founder effect
genetic drift
bottleneck effect

Question 14

Describe genetic drift

Answer 14

Gradual change in allele frequency in a population due to chance
larger effect on smaller populations

Question 15

Describe founder effect

Answer 15

small number of individuals from a large parent population start a new population
only some of the total alleles from the parent population will be present
not all of gene pool present
small pop = more susceptible to effects of genetic drift

Question 16

Describe bottleneck effect

Answer 16

previously large pop suffers dramatic fall in numbers
e.g. natural disasters
reduces population AND genetic diversity & variation
alleles lost -> reduction in gene pool

Question 17

factors that could drive genetic changes in populations

Answer 17

Random mutation
artificial selection
genetic drift / bottleneck / founder effect
migration

Question 18

Explain how small population of species (bottleneck effect) may result in high frequency of rare variant forms

Answer 18

Small population
i) reduced number of alleles, genetic variation, hybrid vigour
ii) a lot of inbreeding, may even lead to inbreeding depression

So, reduced heterozygosity, increased homozygosity, increase in frequency of recessive alleles expressed

Question 19

Genetic consequences of separation of family groups

Answer 19

Lack of hybrid vigour/inbreeding depression
More chance that harmful recessive alleles will be expressed
decrease in hetero/increase in homo
Less genetic variation

Question 20

outline how bacteria become resistant to antibiotics as an example of natural selection

Answer 20

Random mutation in DNA of bacteria
produces allele resistant to antibiotic
antibiotic is selection pressure
bacteria with resistant allele have selective advantage
allowing bacteria to survive and reproduce
passing on resistant allele to offspring
allele frequency increases within the population
carries on and repeats for many generations

Question 21

use the Hardy–Weinberg principle to calculate allele and genotype frequencies in populations

Answer 21

p + q = 1
p^2 + 2pq + q^2 = 1
p = A
q = a
p^2 = AA
2pq = Aa
q^2 = aa

Question 22

state the conditions when the HW principle can be applied

Answer 22

No mutation
no migration
no artificial selection
random mating
large population
no selective pressure against one of the alleles
sexual reproduction
equal allele frequencies in both sexes

Question 23

describe the principles of selective breeding (artificial selection)

Answer 23

humans apply selection pressure
select parents showing desired characteristic
breed parents together (by artificial insemination)
select and breed offspring showing desired characteristic
repeat over many generations
alleles for desired characteristic passed onto offspring
increase in frequency of allele for desired characteristic
implement outbreeding to avoid inbreeding depression

Question 24

Outline the use of selective breeding in the introduction of disease resistance to varieties of wheat
and rice

Answer 24

Humans apply selection pressure
cross parents with desired characteristics
choose & cross offspring showing desired characteristics together
desired characteristic: more grains, higher yield, fast-growing
repeat over many generations
frequency of allele giving desired characteristics increase in population

Question 25

Outline the use of selective breeding in inbreeding and hybridisation to produce vigorous, uniform
varieties of maize

Answer 25

Choose and cross 2 parent maize with desired characteristics
choose and cross the offspring showing desired characteristics
repeat over many generations
desired characteristics - e.g. disease resistance, high yield, more kernels, cold-tolerant
hybridisation - 2 purebred parents - F1 hybrids formed
gives more uniform & vigorous plants

Question 26

Outline the use of selective breeding in improving the milk yield of dairy cattle

Answer 26

choose female with high milk yield
choose male with high milk yield
cross them by means of artificial insemination
choose and cross offspring showing high milk yield
repeat over many generations
alleles for high milk yield passed on to offspring
increase in frequency of allele for high milk yield

Question 27

outline the theory of evolution

Answer 27

Formation of new species
from pre-existing species
over time
due to changes in gene pool (natural selection)
from generation to generation

species shows genetic variation in a population
due to random mutation
selection pressure / competition / predation in the environment
fittest individuals survive & reproduce
allowing them to pass advantageous allele to offspring
increases frequency of advantageous allele in population

Question 28

discuss how AA sequence data can show evolutionary relationships between species

Answer 28

Compare AA sequences of 2 species
more similar = more closely related the 2 species are
more similar = more recent common ancestor/less time elapsed

Question 29

discuss how DNA sequence data can show evolutionary relationships between species

Answer 29

count nucleotide differences
fewer differences = same species
fewer mutations = less time elapsed since most recent common ancestor

Question 30

Suggest why mtDNA is used instead of nuclear DNA when studying the closeness of the relationship between populations

Answer 30

What I understand:
i) Inherited from mother alone, so no recombinants
ii) mtDNA analysis is quicker because a) many mtDNA copies per cell b) fewer genes c) not associated with histones

I don’t understand:
mutations occur at constant rate
mtDNA mutates faster than nuclear DNA
no enzymes to repair DNA mutations

Question 31

Why do those living in same area have similar genetic makeup?

Answer 31

No geographical isolation
Interbreeding occurs
same selection pressure

Question 32

explain how speciation may occur as a result of genetic isolation by geographical separation (allopatric speciation)

Answer 32

geographical isolation/barrier: river, mountain, sea
2 populations separated
different envi cond = diff selection pressures
different mutations
individuals with advantageous alleles SELECTED FOR
more likely to survive & reproduce
advantageous alleles passed on to offspring
change in allele frequency in gene pool -> increase in frequency of advantageous allele within pop.

No gene flow between populations = susceptible to founder’s effect and genetic drift especially with smaller population

Over many generations, leads to different biochemical features = reproductively isolated = allopatric speciation

Question 33

explain how speciation may occur as a result of genetic isolation by ecological and behavioural separation (sympatric speciation)

Answer 33

Sympatric speciation:
Different mutations results in behavioural differences = behavioural isolation = reproductive isolation

No gene flow, gene pool maintained

I don’t understand:
Natural selection
Prezygotic isolating mechanism

Question 34

Examples of Prezygotic isolating mechanism

Answer 34

Temporal isolation: 2 species mate at diff times of year
Ecological isolation: 2 species inhabit similar regions, but occupy different habitats
Behavioural isolation: 2 species respond to diff specific courtship patterns
Mechanical isolation: genital differences prevent copulation

Question 35

Examples of Postzygotic isolating mechanism

Answer 35

Hybrid inviability: hybrids produced, but fail to develop to reproductive maturity
hybrid infertility: hybrids fail to produce functional gametes
hybrid breakdown: F1 hybrids fertile, but F2 generation fail to develop/are infertile

Question 36

Outline differences between artificial and natural selection.

Answer 36

Artificial selection vs natural selection

HUMANS apply selection pressure and for HUMAN’S benefit and does NOT PROMOTE EVOLUTION vs ENVIRONMENT apply selection pressure and for ORGANISM’s benefit and PROMOTES EVOLUTION

Genetic diversity is LOWERED and homozygosity INCREASED and got INBREEDING DEPRESSION vs genetic diversity is HIGHER and homozygosity REDUCED and OUTBREEDING COMMON